A.(1) Field of the Invention
The invention relates to a digital facsimile transmission arrangement comprising a transmitter and a receiver for the transmission of document information, which, in addition to text information also contains photographic information, in the form of a digital signal.
A(2). Description of the Prior Art
Facsimile transmission arrangements have been known for a very long period of time already (approximately 130 years). They have for their object to transmit document information from a transmitter to a receiver. For that purpose the document is divided into document lines having a predetermined line width. Internationally, it has been proposed to opt for such a line width that each millimeter of the document contains 3.85 document lines.
As is known, the transmitter of a facsimile transmission arrangement comprises in its most general form the following components:
a reading arrangement for converting a document line into an analog line signal; PA1 a first coding arrangement for converting an analog line signal into a binary line signal consisting of a sequence of binary elements which have a first or a second value; PA1 a second coding arrangement for coding the binary line signal; PA1 a transmitter output coupled to the output of the second coding arrangement for producing a transmitter output line signal which characterizes the document line. PA1 a receiver input for receiving the transmitter output line signal; PA1 a decoding arrangement coupled to the receiver input and arranged for decoding the received transmitter output line signal; PA1 a reproduction arrangement coupled to the output of the decoder arrangement. PA1 a luminance change detector coupled to the first coding arrangement and arranged for supplying a detection signal when the luminance of the document line has changed by at least a predetermined value; PA1 a first and a second auxiliary coding arrangement to which the first binary (or text) line signal and the second binary (or photo) line signal, respectively is applied, the second auxiliary coding arrangement being controlled by the detection signal and being arranged to convert the sequence of binary elements occurring in said photo-line signal between two consecutive detection signals into a code-word group which characterizes on the one hand the number of binary elements of the first value occurring in said sequence and on the other hand the number of binary elements of the second value occurring in said sequence. PA1 a first and a second auxiliary coding arrangement which each have an input coupled to the receiving arrangement and produce a first and a second binary auxiliary line signal, respectively, the second auxiliary decoding arrangement being arranged to convert each received codeword group into a sequence of binary elements comprising a number of elements of the first value, characterized by this group and a number of elements of the second value, characterized by said group, the elements having the first and the elements having the second value regularly alternating in said sequence; PA1 a reproduction selection circuit for applying selected portions of the first and selected portions of the second binary auxiliary line signal to the reproduction device.
In its most general form the receiver co-operating with said transmitter comprises the following components;
In contemporary embodiments of facsimile transmission arrangements, each document line is divided into a plurality of picture elements. Internationally it has been proposed to divide each document line in, for example, 1728 picture elements. The read arrangement converts each picture element into a signal element which is in the form of a constant signal value which is present during a predetermined period of time .tau. and whose magnitude is proportional to the average luminance of said picture element.
In its most simple form, the first coding arrangement is a threshold circuit which converts every signal element whose signal value is lower than a threshold value into, for example, a binary 1-element and which converts every signal element whose signal value is higher than or equal to said threshold value into a binary 0-element.
It is customary to implement the second coding arrangement as a run-length coding arrangement.
It partitions the binary line signal applied thereto into sub-sequences, in such a manner that each sub-sequence contains only binary elements of the same type and that two consecutive sub-sequences contain elements of different types. Such a sub-sequence is sometimes denoted "run" and the number of binary elements it contains is denoted "run-length". Each run-length is now indicated by a code word. Each codeword is formed from a number of binary elements which in this connection are sometimes called "bits". The number of bits of a codeword is denoted wordlength. This wordlength may be the same for all run-lengths but it is alternatively possible to have the word-length depend on the chance of occurrence of a certain run-length (see reference 1). Said run-length coding arrangement has for its object to convert the binary line signal, which usually consists of 1728 binary elements into a sequence of codewords in such a manner that said codewords together contain a considerably lower number of bits.
In known embodiments of facsimile transmission arrangements this sequence of codewords is at the same time the transmitter output line signal which is transmitted to the receiver. For the transmission said sequence is terminated by an auxiliary codeword which indicates the end of said sequence and at the same time the beginning of the following sequence.
In the receiver which cooperates with this transmitter the received sequence of codewords is applied to the decoding arrangement, which is now in the form of a run-length decoding arrangement. It converts each codeword into a run of binary elements of the same type and thus produces an accurate copy of the binary line signal. This copy is applied to the reproducing arrangement which prints a black picture element at the occurrence of each binary 1-element and a white picture element at the occurrence of each binary 0-element. Reference 2 describes in great detail a reproducing device which has appeared to be very suitable for practical usage.
The facsimile transmission arrangements of the above-described construction appear to be particularly suitable for the transmission of documents which contain only text and/or possibly figures comparable therewith, such as diagrams of electric circuits, or drawings of mechanical constructions. Photographs can only be reproduced with this arrangement with very little detail, the reason being that photographs contain half-tones, while the reproducing arrangement can only produce white and black picture elements.
To render it nevertheless possible to realise with such a reproducing arrangement the picture half-tones necessary for photographs, reference 3 proposes to divide the document into text zones and in photo zones and to apply every portion of the analog line signal relating to a text zone to a text coding arrangement which may be in the form of a threshold circuit, and to apply every portion of the analog line signal relating to a photo zone to a photo coding arrangement. Last-mentioned coding arrangement then incorporates an integrator which is charged by a current whose value is proportional to the instantaneous value of the analog line signal. Each time the voltage across the integrator reaches a predetermined value, said integrator is discharged and a binary 1-element is supplied. The text-coding arrangement produces a binary line signal which will be designated text-line signal and the photo-coding arrangement produces a binary line signal which will be designated photo-line signal.
Thus, in this prior art transmission arrangement, the first coding arrangement is arranged for converting the analog line signal into a first binary line signal, i.e. the text-line signal and into a second binary line signal, i.e. the photo-line signal. As only one of these two line signals, or a combination of both line signals, constitutes an accurate representation of the document line to be transmitted, a selection circuit is also provided for generating a binary document line signal consisting of either the text-line signal or the photo-line signal or of a combination of portions of these line signals. The run-length coded version of this binary document line signal is now applied to the transmitter output as the transmitter output line signal.